Preheat oven to 425 °F (220 °C) and grease a 2‑quart baking dish.

Microwave cauliflower florets in a covered bowl with 1 Tbsp water and a pinch of salt until crisp‑tender (~4–5 min), stirring once. Drain and set aside

Melt 2 Tbsp butter in a skillet over medium; sauté onion and garlic until soft (~5 min). Whisk in flour, salt, pepper, nutmeg, and cayenne; cook ~2 min whisking constantly

Gradually whisk in warmed milk, cook until thickened (~2 min). Remove from heat, stir in Gruyère and half the Parmesan until melted

Stir cauliflower into sauce to coat; transfer mixture to prepared dish

Melt remaining 2 Tbsp butter and combine with Panko, remaining Parmesan, and thyme; sprinkle over gratin top

Bake 25–30 min until topping is golden and sauce bubbles; let rest 10 min before serving